1. Field of the Invention
This invention relates to heterogenous pore polymer nanotube membranes useful in filtration, such as reverse osmosis desalination, nanofiltration, ultrafiltration and gas separation.
2. Description of the Prior Art
Polymer membranes with pores in the polymer surface are often used in reverse osmosis purification of fluids, such as water. These membranes permit transport of water through a solution-diffusion mechanism. A disadvantage is that water must dissolve into the polymer material comprising the membrane and diffuse through. The result is a very low water flux compared to membrane materials that contain open channels through the membrane.
To increase flux in conventional membrane materials, the constituent polmer structure can be made looser by judicious choice of polymerization parameters. However, the rejection performance of the membrane is reduced as a result. In some cases, materials have been added to the polymer to adjust permeability properties. For example, U.S. Pat. No. 4,277,344 (J. E. Cadotte, FilmTec Corporation) describes interfacial synthesis of a reverse osmosis membrane with embedded particles. US patent application no. 2006/007037 (The Regents of the University of California) is a variant of the Cadotte patent, describing methods of membrane fabrication. These membranes remain unsatisfactory since increasing permeability will often reduce selectivity of filtration.
A different type of membrane involves directing water flux through carbon nanotubes attached to a silicon chip. US patent publication no. 2006/033180 (The Regents of the University of California) describes a method of fabricating a material using micromachining or micro electromechanical systems techniques. Holt et al. (Science 312, 1034 (2006)) describes water transport through sub-2 nm inner diameter carbon nanotube pores as being higher than predictions of continuum hydrodynamics models. The method described is a micro-electro-mechanical systems compatible fabrication process for fabrication of carbon nanotube pore membranes using catalytic chemical vapor deposition growth of a dense, vertically-aligned array of double walled carbon nanotubes on the surface of a silicon chip. Gaps between the nanotubes are then filled in a separation step by a process such as vapor deposition. However, this method presents problems with respect to scalability, due to the use of the chemical vapor deposition, and cost, due to the use of silicon as a substrate material.
There remains a need for a scalable filtration membrane that provides adequate flux and selectivity for commercial use in desalination, nanofiltration, and ultrafiltration.